A model study of downstream variations of the thermodynamic structure of the trade winds

A simple model is used to study downstream variations in the thermodynamic structure of the trade-wind boundary layer due to downstream variations in sea-surface temperature, divergence, surface wind speed, and the thermodynamic structure above the inversion. Model results indicate that dry and cold air advection due to an equatorward increase in sea-surface temperature results in a boundary layer that is less than 1 ° cooler and 1 g kg^-1 drier than that obtained for steady-state horizontally homogeneous conditions. Although the magnitude of the advection terms in the heat and moisture budgets may be relatively large, below the inversion they are generally balanced by increased convective heating and moistening. The downstream variations in the temperatures below the inversion are relatively insensitive to variations in any of the external parameters and were generally within 1 °C of the homogeneous solutions. The mixing ratio, however, is sensitive to variations in the surface wind speed and the thermodynamic structure above the inversion.

The height of the trade inversion obtained for the various downstream experiments was found to differ significantly from the horizontally homogeneous solutions. These differences indicate that the height of the inversion may depend more on upstream rather than local conditions. The downstream variations in the inversion height depend on sea-surface temperature, divergence, variations in the temperature and moisture above the inversion, surface wind speeds, and radiative cooling variations.